Objective:
The objectives of our project are to: i) measure N and P loss in runoff from three pastures representing different grazing management strategies over at least 2 years, ii) use the runoff data to validate the ability of our SurPhos model to predict P loss in runoff from grazed pasture, and iii) use SurPhos to simulate annual P loss from 5 Wisconsin grazing farms using producer-collected data.

Approach:
We will establish runoff plots at the USDA-ARS research farm in Prairie Du Sac, WI & install a runoff collection system. We will collect runoff year-round & analyze samples for total sediment, total N and P, & dissolved P, NH4, & NO3. We will collect initial & annual soil samples from plots & have them analyzed. We will stock runoff plots with nonlactating dairy cattle to achieve 3 management scenarios: normal supplementation & grazing intensity, normal supplementation/high grazing intensity, & high supplementation/normal grazing intensity. These strategies will result in different amounts of manure & associated nutrient amounts applied to runoff plots. After grazing periods, we will count the number of dung pats deposited in each runoff plot section & collect samples to determine total wet weight, dry matter content, total P and N, & water-extractable P. We will then quantify nutrient inputs to the runoff plots in cattle feces. We will collaborate with 5 grazing producers in WI whose farms represent a range of stocking densities. We will interview each farm operator quarterly to document farm structure & management, including herd size & composition, livestock facilities, land use, & grazing practices. We will develop grazing & manure spreading logs for producers to document actual manure spreading & grazing practices. These data will enable us to quantify the amount of manure nutrients applied to all pastures. During each visit, we will ask producers for the number of existing cows and their feed management, & take samples of each feed component & pasture grasses for analysis. To determine amount of manure collected & applied to pasture, we will assess herd management, barn cleaning, manure storage practices, & when, where, how, & how much collected manure is land-applied. We will ask where and how long livestock are outdoors to ascertain the amount of uncollected manure. We will collect feces or manure samples from outdoor areas & storage facilities. These data will enable us to determine nutrient mass flows on each farm & the amount of nutrients applied to pastures. Soil samples will be taken from all outside cattle areas for our environmental impact modeling & data validation exercises. We will also collect landscape data, such as slope & slope length to estimate runoff potential in later modeling exercises. Runoff data will be used to validate our SurPhos model, which simulates dissolved P loss in runoff from surface-applied manure. Validation of the model will determine its ability to accurately simulate P loss in runoff from cattle grazing pastures. We are currently adapting SurPhos to simulate grazing and validating the model with field data from Australia & England & small-plot, simulated grazing data from several locations in the U.S. We will use producer information & data to simulate P loss from areas where cows spend time outside, including pasture, over-wintering areas, & barnyards. We will then investigate the physical locations & management practices that represent the greatest risk of P loss, & assess the ability of alternative practices to minimize that loss.